WO2018047955A1

WO2018047955A1 - Case-hardened steel, method for producing same, and method for manufacturing gear part

Info

Publication number: WO2018047955A1
Application number: PCT/JP2017/032544
Authority: WO
Inventors: 佳祐安藤; 岩本　隆; 西村　公宏; 克行一宮
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2016-09-09
Filing date: 2017-09-08
Publication date: 2018-03-15
Also published as: JP2019052376A; MX2019002741A; US11332799B2; CN109689911B; KR20190045314A; KR102279838B1; US20190218633A1; JP6468402B2; JPWO2018047955A1; JP6741060B2; CN109689911A

Abstract

Provided are: a case-hardened steel suitable as a material for manufacturing a mechanical structural part having high rotary bending fatigue strength and impact fatigue strength at relatively low cost; and a method for producing the case-hardened steel. The case-hardened steel contains C, Si, Mn, P, S, Cr, Mo, B, Ti, N, and O in mass% determined by predetermined relationships, contains at least a predetermined amount of Al in relation to the amounts of B, N, and Ti, and contains the remainder comprising Fe and inevitable impurities. Moreover, √I≤80 is satisfied (where I represents the area (μm²) of oxide-based inclusions located at the center of the fisheye in the fractured surface following a rotary bending fatigue test conducted after subjecting the case-hardened steel to carburizing quenching and tempering).

Description

Case-hardened steel, method for producing the same, and method for producing gear parts

The present invention relates to a case hardening steel used as a material for machine structural parts such as automobiles and various industrial machines, a manufacturing method thereof, and a manufacturing method of gear parts. In particular, the present invention relates to a case hardening steel suitable as a material for machine structural parts having high rotational bending fatigue strength and impact fatigue strength, and a method for producing the same.

In recent years, gears used for mechanical structure parts, for example, drive transmission parts such as automobiles, are required to be reduced in size as the vehicle weight is reduced due to energy saving. Therefore, improvement in durability is an issue.

Generally, the durability of a gear is determined by impact fatigue failure of a tooth, rotation bending fatigue failure of a tooth root, and surface pressure fatigue failure of a tooth surface. In particular, in automobile differential gears and the like that are subjected to impact stress, destruction may occur early due to high impact load, so various techniques for improving the impact fatigue strength of case-hardened steel as a material have been studied. ing.

In Patent Document 1, Mo is added to improve the toughness of the carburized layer, and Mn, Cr, and P that lower the grain boundary strength of the carburized layer are reduced, and the lower limit of the value obtained by Mo / (10Si + 100P + Mn + Cr) is set. It is disclosed that the impact characteristics are improved by defining and defining the range of the depth of the carburized hardened layer.

Patent Document 2 discloses that the toughness is improved by controlling the quenching cooling rate range to an appropriate range according to the component composition so that the inside of the gear is a mixed structure of martensite and bainite.

In Patent Document 3, the microstructure is defined in the same manner as Patent Document 2, and the microstructure is a mixed structure of martensite and troostite that improves internal toughness, and the range of addition amounts of Mn and Cr is defined. And the method of suppressing the fall of internal hardness by restrict | limiting Mo addition amount and restrict | limiting the amount of troostite is disclosed.

Patent Document 4 proposes a steel in which Mo is added to the component composition described in Patent Document 3. Patent Document 5 proposes a steel material for bevel gears in which the composite additive amount of Mn, Cr, and Mo is limited in the component composition to suppress the hardness of the steel material and the impact characteristics are improved without impairing the cold forgeability. Yes.

Japanese Patent Publication No.7-100840 Japanese Patent No. 3094856 Japanese Patent No. 3329177 Japanese Patent No. 3733504 Japanese Patent No. 3319648

However, in the method described in Patent Document 1, even if the impact characteristics can be improved, a large amount of Mo, which is an expensive alloy, can be added, or if a large amount of Mo cannot be added, the carburizing time can be greatly extended. Necessary, resulting in a significant increase in product cost or manufacturing cost.

In the method described in Patent Document 2, since the bainite structure is included in the microstructure, it is possible to improve the toughness and increase the impact value. However, if the bainite structure is contained in the internal region of steel, the internal hardness is reduced, so that the gears are easily deformed by impact, and there is a concern that damage will occur if the impact force is repeated.

In the method described in Patent Document 3, since the combined addition amount of Mn and Cr is specified and the addition amount of Mo is regulated, the grain boundary oxidation generated near the surface layer increases, and oxides of Mn and Cr are formed. For this reason, the hardenability is lowered, and an incompletely hardened layer is formed on the surface layer. For this reason, even if the internal hardness can be ensured, the surface layer is liable to break down due to a decrease in the surface layer hardness, and as a result, all fatigue strengths including impact fatigue are reduced.

In the case of the method described in Patent Document 4, even if Mo is added, the hardness inside the gear is reduced due to troostite, so even if the impact characteristics are improved, the fatigue strength such as bending fatigue caused by the inside is reduced. In the case of the method described in Patent Document 5, when the gear is molded by hot forging, the hardness is low and the fatigue strength other than the impact is reduced.

Therefore, in view of the above problems, the present invention provides a case-hardened steel suitable as a material for producing mechanical structural parts having high rotational bending fatigue strength and impact fatigue strength at a relatively low cost, and a method for producing the same. The purpose is to do.

In order to solve the above-mentioned problems, the present inventors diligently studied the effects of components, characteristics after carburizing, and inclusions on the fatigue characteristics after carburizing and tempering. As a result, the following items (A) to (C) were found.
(A) For grain boundary oxide layers that can be the starting point of impact fatigue and bending fatigue cracks, the grain boundary oxide layer grows from the depth direction by adding more than a predetermined amount of Si, Mn, Cr, and Mo. The direction of density increases. Therefore, since there is no oxide layer grown in the depth direction to be the starting point, it is difficult to become a starting point for fatigue cracks.
(B) As described in (A) above, Si, Mn, Cr, and Mo are effective in controlling the grain boundary oxide layer. On the other hand, when excessively added, the amount of retained austenite increases and fatigue cracks occur. Contributes to the generation of Therefore, it is necessary to strictly control the contents of Si, Mn, Cr and Mo.
(C) In order to ensure a solid solution B content of 3 ppm or more that contributes to grain boundary strengthening, the content of each element is strictly determined based on the chemical equilibrium of Ti-Al-BN in steel. Need to control.

The present invention is based on the above findings, and the gist of the present invention is as follows.
[1] By mass%, C: 0.15% to 0.30%, Si: 0.50% to 1.50%, Mn: 0.20% to 0.80%, P: 0.003% to 0.020%, S: 0.005% to 0.050% Cr: 0.30% to 1.20%, Mo: 0.03% to 0.30%, B: 0.0005% to 0.0050%, Ti: 0.002% to less than 0.050%, N: 0.0020% to 0.0150%, and O: 0.0003 % To 0.0025% within the range satisfying the following formula (1),
When Al is [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] ≧ 0.0003%, 0.010% ≦ [% Al] ≦ 0.100% If [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] <0.0003%, (27/14) × {[% N]-(14/48) [% Ti]-(14 / 10.8) [% B] +0.02} ≦ [% Al] ≦ 0.100%,
The balance has a composition composed of Fe and inevitable impurities,
Furthermore, the case hardening steel characterized by satisfying the following formula (2).
Record
1.8 x [% Si] + 1.5 x [% Mo]-([% Mn] + [% Cr]) / 2 ≥ 0.50 (1)
√I ≦ 80 (2)
However, [% M] indicates the content (mass%) of the M element, and I is the fish eye on the fracture surface after carburizing and tempering the case-hardened steel and then performing a rotating bending fatigue test. The area (μm ² ) of the oxide inclusions located in the center is shown.

[2] Skin hardening as described in [1] above, wherein the component composition further includes at least one selected from the group consisting of Nb: 0.050% or less, V: 0.050% or less, and Sb: 0.035% or less. steel.

[3] The case hardening steel according to [1] or [2], wherein the component composition further includes one or more selected from Cu: 1.0% or less and Ni: 1.0% or less in terms of mass%.

[4] One or more selected from the above component composition by mass%: Ca: 0.0050% or less, Sn: 0.50% or less, Se: 0.30% or less, Ta: 0.10% or less, Hf: 0.10% or less The case-hardened steel according to any one of the above [1] to [3], comprising

[5] By mass%, C: 0.15% to 0.30%, Si: 0.50% to 1.50%, Mn: 0.20% to 0.80%, P: 0.003% to 0.020%, S: 0.005% to 0.050% Cr: 0.30% to 1.20%, Mo: 0.03% to 0.30%, B: 0.0005% to 0.0050%, Ti: 0.002% to less than 0.050%, N: 0.0020% to 0.0150%, and O: 0.0003 % To 0.0025% within the range satisfying the following formula (1),
When Al is [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] ≧ 0.0003%, 0.010% ≦ [% Al] ≦ 0.100% If [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] <0.0003%, (27/14) × {[% N]-(14/48) [% Ti]-(14 / 10.8) [% B] +0.02} ≦ [% Al] ≦ 0.100%,
The balance is a steel slab having a composition composed of Fe and unavoidable impurities, subjected to hot working by hot forging and / or hot rolling at a cross-section reduction rate that satisfies the following formula (3), A method for producing a case-hardened steel, characterized by obtaining a case-hardened steel which is a wire.
Record
1.8 x [% Si] + 1.5 x [% Mo]-([% Mn] + [% Cr]) / 2 ≥ 0.50 (1)
(S1-S2) /S1≧0.960 (3)
However, [% M] indicates the content (mass%) of the M element, S1 is the cross-sectional area (mm ² ) of the slab in the cross section orthogonal to the stretching direction during hot working, and S2 is hot The cross-sectional area (mm < ² >) of the said steel bar or wire in the cross section orthogonal to the extending | stretching direction at the time of a process is shown.

[6] Skin hardening as described in [5] above, wherein the component composition further includes at least one selected from the group consisting of Nb: 0.050% or less, V: 0.050% or less, and Sb: 0.035% or less. Steel manufacturing method.

[7] The case-hardened steel according to [5] or [6], wherein the component composition further includes one or more selected from Cu: 1.0% or less and Ni: 1.0% or less by mass%. Production method.

[8] One or more selected from the above component composition by mass%: Ca: 0.0050% or less, Sn: 0.50% or less, Se: 0.30% or less, Ta: 0.10% or less, Hf: 0.10% or less The method for producing a case hardening steel according to any one of the above [5] to [7], comprising:

[9] The case-hardened steel according to any one of [1] to [4] is subjected to machining or forging and subsequent machining to form a gear shape, and then carburizing the case-hardened steel. A method for manufacturing a gear part, comprising: quenching and tempering to obtain a gear part.

[10] In addition to the process for producing the case-hardened steel according to any one of [5] to [8] above, the case-hardened steel is subjected to machining or forging and subsequent machining. A gear part manufacturing method, comprising: carburizing and tempering the case-hardened steel to obtain a gear part.

ADVANTAGE OF THE INVENTION According to this invention, the case hardening steel suitable as a raw material for producing the machine structural components which have high rotational bending fatigue strength and impact fatigue strength at comparatively low cost, and its manufacturing method can be provided. . That is, for example, when a gear is produced as a machine structural component using the steel of the present invention, it is possible to mass-produce gears that are excellent not only in the rotational bending fatigue characteristics of the tooth root but also in the impact fatigue characteristics of the tooth surface. It becomes possible.

It is a figure which shows a rotation bending fatigue test piece. It is a figure which shows the heat processing conditions in a carburizing quenching and tempering process. It is a figure which shows an impact fatigue test piece.

First, the reason why the composition of steel is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding a component shall mean the mass%.

C: 0.15% or more and 0.30% or less In order to increase the hardness of the central part by quenching after carburizing treatment, 0.15% or more of C is required. On the other hand, if the content exceeds 0.30%, the toughness of the core portion decreases, so the C content is limited to a range of 0.15% to 0.30%. Preferably it is 0.15% or more and 0.25% or less of range.

Si: 0.50% or more and 1.50% or less Si increases the resistance to temper softening in the temperature range of 200 to 300 ° C, which is expected to reach during rolling of gears, etc., and also causes residual austenite that causes a decrease in the hardness of the carburized surface layer. It is an element that improves hardenability while suppressing formation. In order to obtain steel having such an effect, addition of at least 0.50% is essential. However, on the other hand, Si is a ferrite stabilizing element, and excessive addition raises the Ac3 transformation point, and ferrite tends to appear in the core portion having a low carbon content in the normal quenching temperature range, resulting in a decrease in strength. . Excessive addition inhibits carburization and causes a decrease in the hardness of the carburized surface layer. In this respect, when the Si content is 1.50% or less, the above-described adverse effects do not occur. From the above, the Si content was limited to the range of 0.50% to 1.50%. Preferably it is 0.80% or more and 1.20% or less of range.

Mn: 0.20% or more and 0.80% or less Mn is an element effective for improving the hardenability, and requires addition of at least 0.20% or more. However, Mn tends to form an abnormal carburization layer, and excessive addition leads to a decrease in hardness due to an excessive amount of retained austenite, so the upper limit was made 0.80%. Preferably it is 0.30% or more and 0.60% or less of range.

P: 0.003% or more and 0.020% or less P is segregated at the grain boundary and causes the carburized layer and the internal toughness to be lowered. Therefore, the lower the amount of P, the better. Specifically, if it exceeds 0.020%, the above-mentioned adverse effects appear, so the P content is set to 0.020% or less. On the other hand, from the viewpoint of manufacturing cost, 0.003% was made the lower limit.

S: 0.005% or more and 0.050% or less S forms a sulfide with Mn and has an effect of improving machinability, so is contained at least 0.005% or more. On the other hand, excessive addition reduces the fatigue strength and toughness of the parts, so the upper limit was made 0.050%. Preferably it is 0.010% or more and 0.030% or less of range.

Cr: 0.30% or more and 1.20% or less Cr is an element effective for improving hardenability, but if its content is less than 0.30%, its additive effect is poor. On the other hand, if it exceeds 1.20%, carburizing abnormal layer It becomes easy to form. Moreover, since hardenability becomes high too much, toughness will deteriorate and fatigue strength will fall. Therefore, the Cr content is limited to the range of 0.30% to 1.20%. Preferably it is 0.40% or more and 0.80% or less of range.

Mo: 0.03% or more and 0.30% or less Mo is an element that has an effect of improving hardenability and toughness and refining the crystal grain size after carburizing treatment, and if less than 0.03%, its addition effect is poor. 0.03% was made the lower limit. On the other hand, if added in a large amount, the amount of retained austenite becomes excessive, which not only causes a decrease in hardness, but also increases the manufacturing cost, so 0.30% was made the upper limit. From the viewpoint of lowering the amount of retained austenite and production cost, the upper limit value is preferably 0.20%.

B: 0.0005% or more and 0.0050% or less B is an element effective for ensuring hardenability by addition of a small amount, and requires addition of at least 0.0005%. On the other hand, if it exceeds 0.0050%, the effect is saturated, so the amount of B is limited to the range of 0.0005% to 0.0050%. Preferably it is 0.0010% or more and 0.0040% or less of range.

Ti: 0.002% or more and less than 0.050% Ti is the element most easily bonded to N and effective for securing the solid solution B, and requires addition of at least 0.002%. However, if it is added excessively, a large amount of hard and coarse TiN is formed, which becomes the starting point of impact fatigue and bending fatigue fracture, and lowers the strength. Since the effect becomes significant at 0.050% or more, the Ti content is limited to the range of 0.002% or more and less than 0.050%. Preferably it is 0.004% or more and less than 0.025% of range. More preferably, it is 0.005% or more and less than 0.025% of range.

N: 0.0020% or more and 0.0150% or less N is an element that combines with Al to form AlN and contributes to the refinement of austenite crystal grains, and requires addition of at least 0.0020% or more. However, if added excessively, not only is it difficult to secure the solid solution B, but also bubbles are generated in the steel ingot during solidification and deterioration of forgeability is caused, so the upper limit is made 0.0150%. Preferably it is 0.0030% or more and 0.0070% or less of range.

O: 0.0003% or more and 0.0025% or less O is an element that exists as an oxide inclusion in steel and impairs fatigue strength. Accordingly, the lower the amount of O, the better, but 0.0025% is acceptable. Preferably it is 0.0015% or less. On the other hand, from the viewpoint of manufacturing cost, 0.0003% was made the lower limit.

The Al content is specified as follows in relation to the B, N, and Ti contents.
[% B]-[(10.8 / 14) x {[% N]-(14/48) [% Ti]}] ≥0.0003%: 0.010% ≤ [% Al] ≤0.100%
Al is an element necessary as a deoxidizer, and at the same time, is an element necessary for ensuring solid solution B in the present invention. Here, [% B] − [(10.8 / 14) × {[% N] − (14/48) [% Ti]}] is the balance obtained by subtracting the amount of B that is stoichiometrically bound to N B amount (hereinafter referred to as [B] amount). If the amount of [B] is 0.0003% or more, it is possible to secure the solid solution B necessary for improving the hardenability. In this case, if the Al content is less than 0.010%, deoxidation becomes insufficient, and the rotational bending fatigue strength and impact fatigue strength are reduced due to oxide inclusions. On the other hand, when Al is added in an amount exceeding 0.100%, the toughness is reduced due to the occurrence of nozzle clogging during continuous casting and the appearance of alumina cluster inclusions. Therefore, when the [B] amount is 0.0003% or more, the Al content is set to a range of 0.010% or more and 0.100% or less.

[% B]-[(10.8 / 14) x {[% N]-(14/48) [% Ti]}] <0.0003%: (27/14) x {[% N]-(14 / 48) [% Ti]-(14 / 10.8) [% B] + 0.02} ≤ [% Al] ≤ 0.100%
On the other hand, when the amount of [B] calculated from the above equation is less than 0.0003%, it is necessary to increase the amount of Al that is relatively easy to bond with N and to secure a solid solution B amount that contributes to improving hardenability. Therefore, Al content is set to (27/14) x {[% N]-(14/48) [% Ti]-(14 / 10.8) [% B] +0.02}% or more to improve hardenability. Ensure a solid solution B content of 0.0003% or more to contribute. Note that the upper limit of Al is 0.100% as described above.

The components in steel in the present invention include the above components, and the balance includes Fe and inevitable impurities, but the following selected components are added for the purpose of imparting other characteristics and the like within a range that does not impair the effects of the present invention. I can do it.

Nb: 0.050% or less Nb is a carbonitride-forming element and contributes to improvement of surface pressure fatigue strength and impact bending fatigue strength by refining the austenite grain size during carburizing. In order to effectively exhibit such an action, when added, the content is preferably 0.005% or more. On the other hand, if it exceeds 0.050%, there is a risk of lowering the coarsening suppression ability and deterioration of fatigue strength due to coarse precipitation of NbC, so the upper limit is preferably made 0.050%. More preferably, it is 0.005% or more and less than 0.025%.

V: 0.050% or less V is a carbonitride-forming element like Nb, and contributes to improving fatigue strength by refining the austenite grain size during carburizing. It also has the effect of reducing the grain boundary oxide layer depth. In order to effectively exhibit such an action, when added, the content is preferably 0.005% or more. On the other hand, the effect is saturated at 0.050%, and if added excessively, coarse carbonitrides are formed, and conversely, the fatigue strength is lowered, so the upper limit is preferably made 0.050%. More preferably, it is 0.005% or more and 0.030% or less of range.

Sb: 0.035% or less Sb has a strong tendency to segregate to grain boundaries, and suppresses grain boundary oxidation of Si, Mn, Cr, etc., which contributes to improving hardenability during carburizing treatment, thereby preventing abnormal carburization in the extreme surface layer of steel. As a result, there is an effect of improving the rotational bending fatigue strength and the impact fatigue strength. In order to effectively exhibit such an action, when added, the content is preferably 0.003% or more. However, adding excessively not only leads to an increase in cost, but also reduces toughness, so 0.035% or less is preferable. More preferably, it is 0.005% or more and 0.020% or less of range.

Cu: 1.0% or less Cu is an element that contributes to the improvement of hardenability. When Cu is added together with Se, it combines with Se in steel and has a useful effect to prevent grain coarsening. It is. In order to obtain these effects, the Cu content is preferably 0.01% or more. On the other hand, when the Cu content exceeds 1.0%, the surface of the rolled material becomes rough and there is a concern that it remains as soot. Therefore, the upper limit is preferably 1.0%. More preferably, it is 0.10% or more and 0.50% or less of range.

Ni: 1.0% or less Ni contributes to improving hardenability and is an element useful for improving toughness. In order to obtain these effects, the Ni content is preferably 0.01% or more. On the other hand, even if it contains exceeding 1.0%, said effect will be saturated. Therefore, the upper limit is preferably 1.0%. More preferably, it is 0.10% or more and 0.50% or less of range.

Ca: 0.0050% or less Ca is a useful element for controlling the form of sulfide and improving machinability. In order to obtain these effects, the Ca content is preferably 0.0005% or more. On the other hand, if the Ca content exceeds 0.0050%, not only the above effect is saturated, but also the formation of coarse oxide inclusions that become the starting point of fatigue failure is promoted, so the upper limit may be made 0.0050%. preferable. More preferably, it is 0.0005% or more and 0.0020% or less of range.

Sn: 0.50% or less Sn is an effective element for improving the corrosion resistance of the steel surface. From the viewpoint of improving corrosion resistance, the Sn content is preferably 0.003% or more. On the other hand, since excessive addition deteriorates forgeability, the upper limit is preferably 0.50%. More preferably, it is 0.010% or more and 0.050% or less of range.

Se: 0.30% or less Se combines with Mn and Cu and is dispersed as precipitates in the steel. Se precipitates exist stably in the carburizing heat treatment temperature range with little precipitate growth, and austenite grain coarsening is suppressed by the pinning effect. For this reason, the addition of Se is effective in preventing coarsening of crystal grains. In order to obtain this effect, it is preferable to add at least 0.001% of Se. On the other hand, even if added over 0.30%, the effect of preventing coarsening of crystal grains is saturated. For this reason, the upper limit is preferably set to 0.30%. More preferably, it is 0.005% or more and 0.100% or less.

Ta: 0.10% or less Ta forms carbides in steel and suppresses the austenite grain coarsening during the carburizing heat treatment by the pinning effect. In order to obtain this effect, it is preferable to add at least 0.003% Ta. On the other hand, if added over 0.10%, cracking is likely to occur during casting solidification, and there is a concern that wrinkles may remain after rolling and forging, so the upper limit is preferably made 0.10%. More preferably, it is 0.005% or more and 0.050% or less of range.

Hf: 0.10% or less Hf forms carbides in the steel and suppresses the coarsening of austenite grains during the carburizing heat treatment by the pinning effect. In order to obtain this effect, it is preferable to add at least 0.003% of Hf. On the other hand, if added over 0.10%, coarse precipitates are produced during casting solidification, which may lead to a decrease in coarsening suppression ability and fatigue strength, so the upper limit is preferably made 0.10%. More preferably, it is 0.005% or more and 0.050% or less of range.
As for the component composition of the case hardening steel of this invention, it is preferable that remainder other than the element demonstrated above consists of Fe and an unavoidable impurity.

When the following (1) formula is satisfied in the case-hardened steel having the above component composition, the present inventors have conventionally provided a machine structural component manufactured by carburizing and tempering the case-hardened steel. It has been found that it exhibits excellent bending fatigue strength and impact fatigue strength.
1.8 x [% Si] + 1.5 x [% Mo]-([% Mn] + [% Cr]) / 2 ≥ 0.50 (1)
However, [% M] indicates the content (mass%) of the M element.

The above equation (1) indicates a factor that affects the grain boundary oxide layer depth. If the value on the left side is less than 0.50, the effect of reducing the grain boundary oxide layer depth is poor. In the present invention, by satisfying the above formula (1), the depth of the carburized grain boundary oxide layer and the low-hardness carburized abnormal layer formed around the grain boundary oxide layer can be reduced. In addition, impact fatigue strength can be improved.

However, even if each element satisfies the above formula (1), the size of oxide inclusions located on the fracture surface of the test piece after the rotating bending fatigue test is larger than a certain value. Further, it has been found that the rotational bending fatigue strength and the impact fatigue strength are reduced due to the oxide inclusions, and thus there is a problem of showing early fatigue failure. Therefore, it is important that the case-hardened steel of the present invention satisfies the following formula (2) after carburizing and quenching. The value of the left side √I of the following formula (2) is more preferably 60 or less, and further preferably 40 or less.
√I ≦ 80 (2)

I on the left side of the above equation (2) is an index indicating the size of the largest oxide inclusions that are the starting points of fatigue fracture, and is obtained as follows. Seven test pieces are collected from the case-hardened steel (bar or wire). The test piece was taken from a position with a diameter of 1/2 in parallel with the drawing direction by hot working (that is, the rolling direction in the case of hot rolling and the drawing direction in forging in the case of hot forging), and is shown in FIG. The parallel part diameter is 8 mm x the parallel part length is 16 mm.

The test piece is subjected to carburizing and tempering under the conditions shown in FIG. 2, and then a double swing Ono type rotating bending fatigue test is performed to cause fish eye fracture. The test conditions are that after carburizing, the surface is polished 0.1 mm, the load stress is 1000 MPa, and the rotational speed is 3500 rpm. In the fatigue test performed by polishing the surface layer in this way, the internal origin failure, that is, the failure starting from inclusions is the main rather than the surface layer failure, and therefore, fish eye failure is observed after the test. Of the seven specimens having the lowest fatigue life, the fracture surface was observed with a scanning electron microscope, and the oxide inclusions located in the center of the fish eye, that is, the largest oxide inclusions The area is measured by image analysis and is defined as I.

According to the method for determining the size of such inclusions in the present invention, the size of the maximum oxide inclusion in a volume of 3.14 × (7.8 mm ÷ 2) ² × 16 mm × 7 = 5349 mm ³ is evaluated. it can. The conventional method for measuring the size, quantity, or density of oxide inclusions in the test area cannot measure the state of oxide inclusions in a large volume, which affects fatigue life. It is not possible to evaluate inclusions that affect In the above-described evaluation method for inclusions in the present invention, the size of oxide inclusions that have actually become the starting point of fatigue fracture of steel in a large volume of 5349 mm ³ can be evaluated. More improved.

Next, a method for producing the case hardening steel according to the present invention will be described.

In order to obtain the case-hardened steel that satisfies the formula (2), in addition to adjusting the component composition of the slab to the above range including the formula (1) in the manufacturing process, the slab On the other hand, it is necessary to perform hot working by hot forging and / or hot rolling at a cross-section reduction rate that satisfies the following expression (3) to form a steel bar or wire.
(S1-S2) /S1≧0.960 (3)
However, S1 is the cross-sectional area (mm ² ) of the slab in the cross section orthogonal to the drawing direction during hot working, and S2 is the cross-sectional area of the steel bar or wire in the cross section orthogonal to the drawing direction during hot working. (Mm ² ).

The left side of the above equation (3) is an index indicating the cross-sectional reduction rate when hot working is performed on the slab. Here, the hot working may be hot forging or hot rolling. Furthermore, both hot forging and hot rolling may be performed. When the index shown on the left side of the above equation (3) is less than 0.960, the rotary bending fatigue strength and the impact fatigue strength are reduced due to the large oxide inclusions, resulting in early fatigue failure. More preferably, the left side of the above formula (3) is 0.970 or more, and more preferably 0.985 or more. In this way, when hot working is performed on a steel slab satisfying the composition of the present invention at a cross-sectional reduction rate that satisfies the above formula (3), the above (2) after carburizing quenching and tempering described later. ) Case-hardened steel satisfying the formula can be obtained.

The case-hardened steel (steel bar or wire) of the present invention produced as described above is subjected to machining such as cutting, with or without hot forging or cold forging, to obtain a component shape (for example, a gear). Shape). Then, a desired part (for example, a gear) is obtained by subjecting this part shape to carburizing quenching and tempering. Further, this part may be subjected to processing such as shot peening. In addition, when hot forging or cold forging is performed during processing, the size of oxide inclusions changes, but it does not change in the direction of worsening fatigue life. Even if it is a case where it becomes a component, it is effective to use the case hardening steel of this invention. The carburizing quenching / tempering conditions for case-hardened steel are not particularly limited, and may be known or arbitrary conditions. For example, the carburizing temperature is 900 ° C. or higher and 1050 ° C. or lower, and the quenching temperature is 800 ° C. or higher. The temperature can be set to 900 ° C. or lower for 10 minutes to 120 minutes, and the tempering temperature 120 ° C. to 250 ° C. for 30 minutes to 180 minutes.

Hereinafter, the configuration and operational effects of the present invention will be described more specifically in accordance with examples. However, the present invention is not limited by the following examples, and can be appropriately changed within the scope that can meet the gist of the present invention, and these are all included in the technical scope of the present invention. It is.

Steel slabs having the composition shown in Table 1 (the content of each element is in mass% and the balance is Fe and inevitable impurities) are hot-rolled at the cross-sectional reduction ratios shown in Table 2, and various dimensions are obtained. A round steel bar was obtained. Steel Nos. 1 to 29 shown in Table 1 are compatible steels whose component compositions satisfy the present invention, and Steel Nos. 30 to 52 are comparative steels whose component compositions do not satisfy the present invention. No. 51 is a comparative example in which the cross-section reduction rate deviates from the regulation value of the present invention.

(Evaluation methods)
The following evaluation was performed on each compatible steel and comparative steel.

(1) Evaluation of rotating bending fatigue strength and I Seven test specimens were collected by the method described above from the position of the diameter 1/2 of each of the round steel bars obtained from the compatible steel and the comparative steel. I was determined. For image analysis, Image-Pro # PLUS manufactured by Media-Cybernetics was used. Table 2 shows the number of repetitions until breakage (the shortest fatigue life out of seven) in the double swing Ono type rotating bending fatigue test in this procedure. In addition, when the shortest fatigue life is 100,000 times or more, it can be regarded as having excellent rotating bending fatigue strength.

(2) Evaluation of impact fatigue strength Samples of 10 × 10 × 110 mm shown in FIG. 3 were collected from each position of the diameter 1/2 of the round bar steel obtained from the conforming steel and comparative steel. did. The obtained test piece was subjected to carburizing and tempering treatment shown in FIG. After that, the impact energy that breaks after 1000 repetitions was investigated using a falling weight type impact fatigue tester. In this test, when it has an impact fatigue strength of 3.5 J or more, it can be regarded as having an excellent impact fatigue strength. The evaluation results are shown in Table 2.

ADVANTAGE OF THE INVENTION According to this invention, the case hardening steel suitable as a raw material for producing the machine structural components which have high rotational bending fatigue strength and impact fatigue strength at comparatively low cost, and its manufacturing method can be provided. .

Claims

In mass%, C: 0.15% to 0.30%, Si: 0.50% to 1.50%, Mn: 0.20% to 0.80%, P: 0.003% to 0.020%, S: 0.005% to 0.050%, Cr : 0.30% to 1.20%, Mo: 0.03% to 0.30%, B: 0.0005% to 0.0050%, Ti: 0.002% to less than 0.050%, N: 0.0020% to 0.0150% and O: 0.0003% to 0.0025 % Or less under a range that satisfies the following formula (1):
When Al is [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] ≧ 0.0003%, 0.010% ≦ [% Al] ≦ 0.100% If [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] <0.0003%, (27/14) × {[% N]-(14/48) [% Ti]-(14 / 10.8) [% B] +0.02} ≦ [% Al] ≦ 0.100%,
The balance has a composition composed of Fe and inevitable impurities,
Furthermore, the case hardening steel characterized by satisfying the following formula (2).
Record
1.8 x [% Si] + 1.5 x [% Mo]-([% Mn] + [% Cr]) / 2 ≥ 0.50 (1)
√I ≦ 80 (2)
However, [% M] indicates the content (mass%) of the M element, and I is the fish eye on the fracture surface after carburizing and tempering the case-hardened steel and then performing a rotating bending fatigue test. The area (μm 2 ) of oxide inclusions located in the center is shown.
The case hardening steel according to claim 1, wherein the component composition further includes at least one selected from the group consisting of Nb: 0.050% or less, V: 0.050% or less, and Sb: 0.035% or less in terms of mass%.
The case-hardened steel according to claim 1 or 2, wherein the component composition further includes one or more selected from Cu: 1.0% or less and Ni: 1.0% or less in terms of mass%.
The component composition further includes one or more selected from Ca: 0.0050% or less, Sn: 0.50% or less, Se: 0.30% or less, Ta: 0.10% or less, and Hf: 0.10% or less in mass%. Item 4. The case-hardened steel according to any one of items 1 to 3.
In mass%, C: 0.15% to 0.30%, Si: 0.50% to 1.50%, Mn: 0.20% to 0.80%, P: 0.003% to 0.020%, S: 0.005% to 0.050%, Cr : 0.30% to 1.20%, Mo: 0.03% to 0.30%, B: 0.0005% to 0.0050%, Ti: 0.002% to less than 0.050%, N: 0.0020% to 0.0150% and O: 0.0003% to 0.0025 % Or less under a range that satisfies the following formula (1):
When Al is [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] ≧ 0.0003%, 0.010% ≦ [% Al] ≦ 0.100% If [% B]-[(10.8 / 14) × {[% N]-(14/48) [% Ti]}] <0.0003%, (27/14) × {[% N]-(14/48) [% Ti]-(14 / 10.8) [% B] +0.02} ≦ [% Al] ≦ 0.100%,
The balance is a steel slab having a composition composed of Fe and unavoidable impurities, subjected to hot working by hot forging and / or hot rolling at a cross-section reduction rate that satisfies the following formula (3), A method for producing a case-hardened steel, characterized by obtaining a case-hardened steel which is a wire.
Record
1.8 x [% Si] + 1.5 x [% Mo]-([% Mn] + [% Cr]) / 2 ≥ 0.50 (1)
(S1-S2) /S1≧0.960 (3)
However, [% M] indicates the content (mass%) of the M element, S1 is the cross-sectional area (mm 2 ) of the slab in the cross section perpendicular to the stretching direction during hot working, and S2 is hot The cross-sectional area (mm < 2 >) of the said steel bar or wire in the cross section orthogonal to the extending | stretching direction at the time of a process is shown.
The component composition is further in terms of mass%, Nb: 0.050% or less, V: 0.050% or less, and Sb: 0.00.
The method for producing a case hardening steel according to claim 5, comprising one or more selected from 035% or less.
The method for producing a case-hardening steel according to claim 5 or 6, wherein the component composition further includes at least one selected from Cu: 1.0% or less and Ni: 1.0% or less in terms of mass%.
The component composition further includes one or more selected from Ca: 0.0050% or less, Sn: 0.50% or less, Se: 0.30% or less, Ta: 0.10% or less, and Hf: 0.10% or less in mass%. Item 8. The method for producing case-hardened steel according to any one of Items 5 to 7.
The case-hardened steel according to any one of claims 1 to 4 is subjected to machining or forging and subsequent machining to form a gear shape, and then carburized and quenched and tempered to the case-hardened steel. A method for manufacturing a gear part, comprising obtaining a gear part.
In addition to the method of manufacturing the case-hardened steel according to any one of claims 5 to 8, the case-hardened steel is subjected to machining or forging and subsequent machining to obtain a gear shape, and thereafter A gear part manufacturing method, comprising: carburizing and tempering the case-hardened steel to obtain a gear part.